[proxies] do not leak private symbols to proxy traps

test/mjsunit/harmony/proxies-delete-property.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --harmony-proxies --harmony-reflect
+// Flags: --harmony-proxies --harmony-reflect --allow-natives-syntax
 
 
 var properties =
     ["bla", "0", 1, Symbol(), {[Symbol.toPrimitive]() {return "a"}}];
 
 
 function TestForwarding(handler, myDelete, shouldThrow) {
   var target = {};
   var proxy = new Proxy(target, handler);
 
@@ skipping 165 matching lines @@
 
 
 TestTrappingFalsish(
     (o, p) => delete o[p], false);
 TestTrappingFalsish(
     (o, p) => Reflect.deleteProperty(o, p), false);
 TestTrappingFalsish(
     (o, p) => {"use strict"; return delete o[p]}, true);
 TestTrappingFalsish(
     (o, p) => {"use strict"; return Reflect.deleteProperty(o, p)}, false);
+
+(function testDeletePrivateSymbol() {
+  var symbol = %CreatePrivateSymbol("secret");
+  var O = { [symbol]: "value" };
+  var proxy = new Proxy(O, { delete(t, name) { assertUnreachable(); }});
+  delete proxy[symbol];
+  assertEquals(true, Reflect.deleteProperty(proxy, symbol));
+  assertEquals("value", O[symbol]);
+  assertEquals(true, Reflect.deleteProperty(O, symbol));
+  assertEquals(undefined, O[symbol]);
+})();